Concerted action of Msx1 and Msx2 in regulating cranial neural crest cell differentiation during frontal bone development

The homeobox genes Msx1 and Msx2 function as transcriptional regulators that control cellular proliferation and differentiation during embryonic development. Mutations in the Msx1 and Msx2 genes in mice disrupt tissue–tissue interactions and cause multiple craniofacial malformations. Although Msx1 and Msx2 are both expressed throughout the entire development of the frontal bone, the frontal bone defect in Msx1 or Msx2 null mutants is rather mild, suggesting the possibility of functional compensation between Msx1 and Msx2 during early frontal bone development. To investigate this hypothesis, we generated Msx1−/−;Msx2−/− mice. These double mutant embryos died at E17 to E18 with no formation of the frontal bone. There was no apparent defect in CNC migration into the presumptive frontal bone primordium, but differentiation of the frontal mesenchyme and establishment of the frontal primordium was defective, indicating that Msx1 and Msx2 genes are specifically required for osteogenesis in the cranial neural crest lineage within the frontal bone primordium. Mechanistically, our data suggest that Msx genes are critical for the expression of Runx2 in the frontonasal subpopulation of cranial neural crest cells and for differentiation of the osteogenic lineage. This early function of the Msx genes is likely independent of the Bmp signaling pathway.